Don't worry about it if they go missing. Kids will loose things and that does not mean you are not watching her. I hope the parent's don't expect you to keep your eyes on their daughter the whole time she is at your house! Maybe ask mom for non-slip pony tail's for her instead to keep her hair back.

I have an all girl group right now and I am constantly fixing their hair - clips, ponytails, headbands. Things get lost - oh well!

I would be more concerned that another child will put the clip in their mouth.